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Boiler slag is a by-product produced from a wet-bottom boiler, which is a special type of boiler designed to keep bottom ash in a molten state before it is removed. These types of boilers (slag-tap and cyclone boilers) are much more compact than pulverized coal boilers used by most large utility generating stations and can burn a wide range of fuels and generate a higher proportion of bottom ash than fly ash (50 to 80% w/w bottom ash compared to 15 to 20% w/w bottom ash for pulverized coal boilers). With wet-bottom boilers, the molten ash is withdrawn from the boiler and allowed to flow into quenching water. The rapid cooling of the slag causes it to immediately crystallize into a black, dense, fine-grained glassy mass that fractures into angular particles, which can be crushed and screened to the appropriate sizes for several uses.

There are two types of wet-bottom boilers: (i) the slag-tap boiler and (ii) the cyclone boiler. The slag-tap boiler burns pulverized coal, and the cyclone boiler burns crushed coal. In each type, the bottom ash is kept in a molten state and tapped off as a liquid. Both boiler types have a solid base with an orifice that can be opened to permit the molten ash that has collected at the base to flow into the ash hopper below. The ash hopper in wet-bottom furnaces contains quenching water. When the molten slag comes in contact with the quenching water, it fractures instantly, crystallizes, and forms pellets. The resulting boiler slag, often referred to as black beauty, is a coarse, hard, black, angular, glassy material.

Since boiler slag is angular, dense, and hard, it is often used as a wear-resistant component in surface coatings of asphalt in road paving. Finer-sized boiler slag can be used as blasting grit and is commonly used for coating roofing shingles. Other uses include raw material for the manufacture of cement and in colder climates; it is spread onto icy roads for traction control. Because there are so many uses and such a limited supply, most of the boiler slag produced in the United States is used and some even imported from other countries.

See also: Ash, Biomass Ash.